This invention relates to an optical filter system and more particularly to a sharp transition, high precision cholesteric liquid crystal combination filter for blocking the transmission of spurious, nearly visible infrared emissions in a night vision system.
It is well known that a pilot in an attack aircraft uses night vision goggles that permit the pilot to see infrared radiation from targets or other objects outside the aircraft.
Unfortunately, useful equipment within the cockpit may emit spurious infrared light at levels significant enough to cause problems for the pilot. For example, displays located in the cockpit area for generating red colored symbology also emit spurious infrared light at levels high enough to blind the pilot's night vision goggles to low level infrared sources of interest. In many cases the spurious infrared light has a wavelength that is almost in the visible region. Thus, it is highly desirable in night vision applications to provide a simple means for blocking spurious, nearly visible infrared emissions while permitting the maximum transmission of visible light at adjacent wavelengths.
Conventional infrared filters are known which block the transmission of light at infrared wavelengths. However, these filters lack the sharp transmission step characteristic needed to block transmission of nearly visible infrared light while also permitting maximum transmission of all visible light up to the infrared range.
Conventional head-up displays, night vision systems, and known optical devices do not teach or suggest any arrangement for achieving such a sharp transition high precision filter.
For example, Jacobs, et al. U.S. Pat. No. 4,679,911, teaches the use of cholesteric liquid crystal materials to shape the profile of an optical beam, e.g., a laser. U.S. Pat. No. 4,679,910 to Afron, et al teaches conversion of visible images to infrared using liquid crystal light valves, U.S. Pat. No. 4,423,927 to Bly describes a bandpass filter using twisted nematic liquid crystal devices having different time responses and opposite rotary direction handedness. U.S. Pat. No. 4,394,069 to Kay describes a liquid crystal tuned filter using zero-twist liquid crystal cells to obtain a narrow band transmission characteristic. U.S. Pat. Nos. 4,232,948 and 4,416,514, although pertaining to optical systems, do not teach or suggest any optical filter system capable of transmitting light up to an abrupt wavelength.
It is known that cholesteric liquid crystal optical filters are capable of transmitting light at substantially all wavelengths while reflecting light over a single, generally narrow, wavelength band. For example, U.S. Pat. No. 3,679,290 to Adams et al discusses the use of a matched pair of cholesteric liquid crystal elements to form an optical notch filter. The notch filter consists of one element that reflects right-hand circularly polarized light near a given wavelength and transmits left-hand circularly polarized light, and a second film that reflects left-hand circularly polarized light wavelength and transmits right-hand circularly polarized light. The two elements are arranged in series such that the filter effectively transmits all incident light with the exception of the wavelength band centered around that nominal wavelength. See also, U.S. Pat. No. 3,711,181 to Adams et al.
It is also known that the unique optical properties of cholesteric liquid crystal elements can be exploited to provide a wide variety of narrow band filtering functions extending over a wide wavelength range from the near ultraviolet to the far infrared. For example, an article by Adams, et al entitled "Cholesteric Films as Optical Filters," Journal of Applied Physics, Vol. 42, No. 10, September 1971, discloses several cholesteric element configurations which provide a notch filter function.
Although the foregoing Adams references discuss the general properties of cholesteric liquid crystal filters, these references are largely theoretical in nature and do not teach or suggest the use of the discussed cholesteric liquid crystal filters in sophisticated, practical applications.
Moreover, it is known that liquid crystal filters suffer from a number of limitations. First, it is known that the performance of cholesteric liquid crystal filters drifts over temperature. Therefore, if two elements are matched at one temperature, they may drift apart in performance at another temperature. Moreover, cholesteric liquid crystal filters have been previously used in essentially narrow band filtering applications.
Thus, although the use of cholesteric liquid crystal filters is known, the use of these filters in high precision, night vision, or in wideband filtering applications is neither taught nor suggested by the art.